The present invention relates to furnaces in general, especially to industrial furnaces for the burning of refuse or the like, and more particularly to improvements in a method and furnace for combustion of secondary (normally lower-quality) fuels simultaneously with primary fuels, especially for combustion of unprocessed or partially processed viscous secondary fuels having the consistency of mud or slime with a primary fuel which may constitute refuse or a conventional fuel (such as wood, coal or the like).
It is known to feed finely comminuted sludge which is removed from settling tanks into the ascending current of gaseous combustion products in an industrial furnace. The level of the locus of admission of finely comminuted sludge is selected in such a way that a certain percentage of descending sludge particles is fully relieved of moisture and the remainder of the particles is likely to be subjected to partial drying before the particles reach the burning layer of primary fuel on the grate. The descending particles are dried by a portion of hot gases which rise above the grate; to this end, such portion of the gases is segregated from the remaining hot gases and is fed into the path of descending sludge particles which are normally caused to cover a substantial distance prior to reaching the grate.
The just described conventional method exhibits a number of drawbacks. Thus, the particles which are completely dried before they reach the grate are entrained by the ascending gases and are not combusted at all or are oxidized outside of the combustion chamber. Furthermore, relatively large particles are dried only in the region of their exposed surfaces. When a partially dried particle reaches the grate and happens to come to rest on a mass of partially combusted fuel, its outermost layer is converted into coke while the core remains uncombusted. The particle is thereupon caused to leave the combustion chamber together with the slag. Attempts to prevent such partial combustion of sludge particles include the provision of complex and expensive devices which insure that the size of all comminuted particles is within a rather narrow range, i.e., a range which guarantees pronounced reduction of moisture content of each and every particle and complete combustion of a high percentage of dried particles on their way toward the layer of primary fuel on the grate. Such comminution of sludge can be achieved only with substantial expenditures in energy. Moreover, secondary fuel having a muddy consistency cannot be readily comminuted with a sufficient degree of predictability so that the descending shower of particles of secondary fuel invariably contains a relatively high percentage of larger particles which undergo partial combustion prior to evacuation from the combustion chamber.
In accordance with certain other presently known proposals, the period of dwell of particles of sludge in a hot atmosphere is prolonged to such an extent that all or nearly all particles are adequately dried prior to descending onto the burning layer of primary fuel. A drawback of such proposals is that the rate of admission of secondary fuel cannot be regulated with a requisite degree of predictability.